Can an astronaut , on a spacewalk, open his face mask?

Only once.

I wouldn’t be surprised if there is some kind of interlock that would prevent opening when there is that kind of pressure differential.

If you insist, you would have an awful choice to make… release pressure suddenly so you can get to the itch fast then spend less time not breathing but probably wreaking havoc on eardrums.
Or release pressure slowly but not have air to breathe for a long time (could you hold your breath vs vacuum? hmm..)
The collateral damage to skin and ears and eyes would be pretty bad, I think.

He’d have about ten seconds provided he remembered to exhale and kept his mouth closed.

He’d get a nasty sunburn for his trouble, best case scenario.

Astronauts have a piece of Velcro inside their helmet to scratch their nose.

The pressure differential from taking off the helmet would most likely cause an aneurism pretty quickly.

@Seek That’s why you turn with your back to the Sun…. and get wicked frostbite instead.

Hum, that would be an interesting way to go

That’s what the “Dolly Parton” is for: http://www.newsweek.com/quora-question-how-astronauts-deal-itchy-body-part-while-inside-protective-323060